When doe's a boat become a ship?

A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat". Consequently submarines are referred to as "boats", because early submarines were small enough to be carried aboard a ship in transit to distant waters. Another type of large vessel which is traditionally called a boat is the Great Lakes freighter. Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship (cf. vessel).

During the age of sail, ship signified a ship-rigged vessel, that is, one with three or more masts, usually three, all square-rigged. Such a vessel would normally have one fore and aft sail on her aftermost mast which was usually the mizzen. Almost invariably she would also have a bowsprit but this was not part of the definition.
This came right out of wikipedia and is still somewhat ambiguous. In the US a vessel under 65 ft is considered a "small craft". There are plenty of small craft that carry dinghys or life rafts and ply the oceans. There are private yachts that are over 250 ft in length, the record is about 450 for a Saudi royal family.

A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat".

Boats don't become ships. A boat is a small vessel usually with oars or paddle.(Like a rowboat) Add a motor and it becomes a motorboat. Add a sail and it becomes a sailboat. A ship is an ocean-going vessel.
Hence, boats don't navigate in oceans and ships don't navigate on lakes, etc. Does that help?